Arginase (L-arginase, L-arginine amidino hydrolase: E. C. 3.5.3.1) is an enzyme which has been known for more than 50 years and which catalyzes the enzymatic production of L-ornithine from L-arginine. This catalysis takes place in vivo in the liver of mammals in the last stage of the urea cycle, during which urea and L-ornithine (2,5-diaminopentanoic acid) are formed by hydrolysis from (L-arginine 2-amino-5-guanidino pentanoic acid). The enzyme can correspondingly be obtained from mammal liver, e.g. calf's liver; however, it also occurs in the flora kingdom as well as in several microorganisms.
Arginase has a molecular weight of approximately 138,000 and consists of 4 identical subunits. Mn.sup.2+ as well as Co.sup.2+ and Ni.sup.2+ can be added as typical activator.
L-ornithine is an amino acid which occurs in the body of mammals but is not produced during anabolism and is therefore not incorporated in proteins, that is, a natural but non-proteinogenic amino acid.
L-ornithine can replace L-arginine, an amino acid essential in infants and children, in all functions. Since the salts of L-ornithine entail a lesser stressing with urea of the organism and exhibit in part a better solubility behavior than L-arginine, L-ornithine has considerable commercial potential. A deficiency of arginine or ornithine can result in injury, in some cases in death, e.g. by means of an elevated ammonia level on account of high amino acid adsorption after a period of fasting or malnutrition.
Arginase has long been used as a diagnostic enzyme. However, this enzyme could not be used hitherto for the production of L-ornithine from L-arginine on an industrial scale since it exhibits only a very slight stability under reaction conditions and correspondingly can not be recovered or can be recovered only to a slight extent from an enzyme batch. Therefore, because of the high enzyme expense, the enzymatic production of L-ornithine from L-arginine is not cost effective on an industrial scale.
Therefore, the only potential candidates for industrial production of L-ornithine and its salts have included only fermentation from glucose with strains of Brevibacterium, Corynebacterium and Arthobacter, and the chemical hydrolysis of L-arginine in addition to the enzymatic method. However, the chemical hydrolysis results in many byproducts, e.g. in the partial hydrolysis to L-citrulline (2-amino-5-ureido pentanoic acid) or in the racemization of L-arginine or L-ornithine. Fermentation to L-ornithine is economical only in high tonnages.
Although arginase exhibits a satisfactory activity and selectivity for the hydrolysis of L-arginine to L-ornithine, the stability of the enzyme for industrial use is insufficient. In order to obtain good activity, the addition of bivalent manganese ions in addition to the enzyme to the reaction solution is necessary. However, when the reaction is carried out at the generally adjusted pH of the reaction of 9.5, which corresponds to the activity optimum of arginase published in the literature, the oxidation of bivalent manganese to tetravalent manganese frequently causes precipitatation of manganese dioxide after a brief time. A deactivation of the arginase also occurs (M. Munakata et al., Bioinorganic Chemistry 1976, 6, pp. 133-42; V. Rossi et al., Int. J. Peptide Protein Res. 1983, 22, pp. 239-50).